1. Field of the Invention
The present invention relates to a valve timing adjusting apparatus, which variably controls the timing for opening and/or closing at least one of an intake valve and an exhaust valve in accordance with the driving state of the internal combustion engine.
2. Description of the Related Art
As a conventional valve timing adjusting apparatus for internal combustion engine, there has been disclosed a device that comprises a casing integrally provided with a timing sprocket or a timing pulley, to which a rotational force is transmitted from a crank shaft of the internal combustion engine (hereinafter may be referred to just as a "combustion engine"), a rotor which is connected to a camshaft residing in the control system side for controlling the timing for opening and/or closing the intake valve and/or the exhaust valve of the combustion engine and is accommodated in the casing, and a bias means for urging an unidirectional rotational force to the rotor, wherein the timing for opening and/or closing the intake valve and/or the exhaust valve is adjusted by controlling the oil pressure in the hydraulic chambers formed between a plurality of shoes of the casing and vanes of the rotor, and changing thereby the relative rotational phase difference between the casing and the rotor.
In the configuration above, the bias means urges the rotor with an urging force which is set to the level equal to or greater than the maximum inertia torque or the average inertia torque of the camshaft at the time of starting of the combustion engine, and the rotor is maintained at the most advanced position by this urging force when the combustion engine comes to a halt. In this way, due to the fact that the rotor is maintained at the most advanced position when the combustion engine has come to an inactive state, the valve open period of the intake valve and that of the exhaust valve will never exist simultaneously, so that the combustion gas is prevented from returning, and the combustion engine can thus be driven properly.
As the reference documents disclosing the conventional valve timing adjusting apparatus for internal combustion engine, there are a document titled "Kokai Giho" by "Hatsumei Kyokai" with the volume number 87-8631, Japanese Patent Application Laid-Open No. 10-68306, Japanese Patent Application Laid-Open No. 9-264110 and so on. In these documents, the former document "Kokai Giho" discloses such technique that an urging force is supplied in a specific predetermined direction by providing a spring between the rotor and the casing, whereas the latter patent application documents disclose a device, which is provided with a most advanced position holding (or locking) mechanism, and supplies an urging force in the advance direction, wherein the thus supplied urging force is set to the level equal to or greater than the maximum torque or the average torque at the starting time of the combustion engine.
As the conventional valve timing adjusting apparatus for internal combustion engine is configured as explained above, an urging force of the bias means urging the rotor is set to the level greater than the maximum inertia torque or the average inertia torque at the starting time of the combustion engine, and the rotor is urged toward the advance side with respect to the casing by the urging force, so that in a case that the relative phase difference between the casing and the rotor is varied toward the direction opposite to the direction of the urging force by the oil pressure supplied to the hydraulic chambers formed between shoes of the casing and vanes of the rotor, the response characteristic with respect to the relative rotations of the casing and the rotor is deteriorated due to the effect of the urging force, and thus the operation speed is also lowered.